Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates. Basically, a cathode assembly including the sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits on the receiving surface of the substrate to form a thin layer or film.
A wide variety of metallic and ceramic materials have been used to form sputter targets for use in the manufacture of electrical components and other industrial products. Tantalum (Ta) sputter targets are used, for example, for forming tantalum or tantalum-nitrogen barrier layers in the metallization of substrates for ultra large-scale integrated circuits.
A sputter target itself is essentially a metal or ceramic plate, the cross-sectional shape and surface contour of which varies with different sputtering applications. It has been found that the microscopic structure of the material comprising the sputter target has a significant impact on, among other performance criteria, the sputtering rate and the uniformity of the film deposited during the sputtering process.
A metallic sputter target typically comprises a plurality of "grains" of a size visible under an optical microscope. Within each grain, the metal atoms align in a crystalline matrix. The orientations of the crystalline matrices in the various grains are referred to as the "textures" of the grains. Where the crystalline matrices of different grains in a metal sample are oriented in different directions, the sample as a whole is referred to as having a "random texture."
Pure tantalum is a Group Vb transition metal which typically crystallizes in a "body-centered cubic" matrix. As shown schematically in FIG. 1, each tantalum atom 10 in the body-centered cubic matrix (or, more accurately, in a "unit cell" 12 of the matrix) is typically surrounded by eight other equally spaced tantalum atoms 14. Two sets of directions relative to the crystalline matrix are specifically indicated on the unit cell 12 of FIG. 1: that indicated by the arrows 16 along "sides" of the unit cell 12 (referred to collectively as "&lt;200&gt; directions") and that indicated by the arrows 18 along "diagonals" of the unit cell 12 (referred to collectively as "&lt;222&gt; directions").
Since the unit cell 12 is symmetric along all three &lt;200&gt; directions, each of the &lt;200&gt; directions is physically equivalent to each of the other &lt;200&gt; directions . Likewise, each of the &lt;222&gt; directions is physically equivalent to each of the other &lt;222&gt; directions. Note that the atoms 10, 14 in the unit cell 12 are more closely packed along the &lt;222&gt; directions than along the &lt;200&gt; directions.
High purity tantalum is often melted using an electron-beam technique and then cast to form ingots or billets having random textures and grain sizes ranging to the order of several centimeters. One conventional method for fabricating tantalum targets uses side-forging and side-rolling to reduce the thicknesses of the billets along their centerlines, These processes typically induce grains which are elongated in the direction of the billet centerline. The resulting metal has either a random textures or a predominantly &lt;200&gt; textures, in which a majority of the grains are aligned so that one of their &lt;200&gt; directions is normal to the forging or rolling plane.
Alternatively upset forging and upset rolling have been used in the fabrication of tantalum sputtering targets. In these processes, one of the dimensions of the ingot is reduced while one or more of the other dimensions are allowed to increase.
In the past, the high purity tantalum metal used for the fabrication of sputter targets has been only lighty worked. Dimensional reductions of about 50% to about 80% are typical in conventional side-forging and side-rolling processes. Since the billets have not been heavily worked, the conventional processes have produced targets having large, non-uniform -rain sizes ranging from about 70 .mu.m to about 300 .mu.m. Since the metal is only lightly worked, high temperatures, on the order of 1500.degree. C., are required to fully anneal the billets.
It has been found that targets with predominatly &lt;222&gt; texture have higher sputtering rates and deposit more uniform metallic films on substrates during conventional sputtering processes. Without wishing to be bound by any theory of operation, predominantly &lt;222&gt; texture is believed to produce better results due to the closer packing of the atoms along the &lt;222&gt; directions of the crystalline matrices. It has also been found that targets having smaller grain sizes tend to have have higher sputtering rates and to deposit more uniform metallic films.
Accordingly, there remains a need for a method for fabricating tantalum sputtering targets having predominantly &lt;222&gt; texture and small, uniform grain sizes on the order of 20-25 .mu.m.